In 3GPP Long Term Evolution (LTE) and 3GPP Long Term Evolution-Advanced (LTE-A), with a continuous and rapid increase in a quantity of antennas at a data transmit end, a quantity of user equipments (UE) that need to be served, that is, a quantity of to-be-scheduled UEs, also rapidly increases. The increase in the quantity of antennas can provide higher spatial degrees of freedom. This creates a favorable condition for multiplexing multiple data streams in downlink space, for example, for multiple-input multiple-output (MIMO), including single user multiple-input multiple-output and multi-user multiple-input multiple-output.
To obtain high spatial degrees of freedom that can be provided by massive antennas, the data transmit end (for example, a base station) needs to obtain related channel state information (CSI), so as to obtain a precise precoding matrix. In MIMO, the data transmit end generally obtains the CSI by using two methods in the prior art (for example, LTE/LTE-A):
In one method, in a case of time division duplex (TDD) or frequency division duplex (FDD), the data transmit end sends a pilot for downlink CSI measurement, a data receive end (for example, UE) measures the pilot to obtain CSI, the UE then feeds back the CSI, and the data transmit end performs precoding on data by using the CSI and sends precoded data. The CSI that is fed back by the UE is generally quantized CSI, and is a precoding matrix indicator (PMI) and a rank indicator (RI) in LTE. In the other method, in a case of TDD, a data receive end sends a pilot for uplink CSI measurement, for example, a sounding reference signal (SRS) in LTE/LTE-A, the data transmit end performs uplink channel CSI measurement, and the data transmit end considers, according to channel reciprocity, that uplink channel measurement is equivalent to downlink channel measurement (a necessary reciprocity parameter is generally required for calibration), and then performs precoding on data according to the CSI and sends precoded data.
Currently, a key to obtaining CSI in MIMO is to rapidly obtain complete real-time CSI (completeness refers to channel information of all antennas at a data transmit end) at a time, so as to obtain a most accurate channel as much as possible. Only a level-1 reference signal (RS) needs to be used to obtain the CSI. Specifically, the level-1 RS may have the following two implementations:
Implementation 1: As shown in FIG. 1, a data transmit end first sends a pilot for downlink CSI measurement, for example, a CSI-RS or a cell-specific reference signal (CRS) in LTE/LTE-A, a data receive end measures the pilot to obtain CSI (for example, a precoding matrix in LTE/LTE-A) and performs quantization on the CSI, the data receive end feeds back quantized CSI to the transmit end, and the transmit end performs precoding on data by using the CSI and sends precoded data. This solution is applicable to FDD and TDD.
Implementation 2: As shown in FIG. 2, a data receive end transmits a pilot for uplink CSI measurement, for example, an SRS in LTE/LTE-A, and a data transmit end measures the pilot to obtain CSI, for example, a precoding matrix in LTE/LTE-A, and then performs precoding on data according to the CSI and sends precoded data. This solution is applicable to TDD.
An amount of downlink pilot overheads is directly proportional to a quantity of antennas at a data transmit end, an amount of uplink pilot overheads are directly proportional to a quantity of to-be-served UEs, and a quantity of uplink CSI feedbacks is also directly proportional to the quantity of antennas at the data transmit end. Therefore, when the quantity of antennas at the data transmit end is not quite large (for example, 4/8 antennas in LTE/LTE-A), pilot overheads and the quantity of uplink CSI feedbacks can be controlled. However, when the quantity of antennas at the data transmit end is relatively large (a quantity of UEs that can be scheduled also increases), uplink and downlink pilot overheads and the quantity of uplink CSI feedbacks occupy a large quantity of time-frequency resources. As a result, time-frequency resources available for data transmission are reduced, and a system throughput is greatly affected.
In the prior art, an array whose dimension is a quantity of antennas at a data transmit end needs to be processed. This poses an unprecedented challenge to a baseband processing capability of the data transmit end. Both a CSI feedback using downlink measurement and uplink CSI measurement easily cause excessive pilot overheads for obtaining CSI and excessive computation for obtaining channel information when there is a relatively large quantity of antennas at a transmit end.